This invention relates to a device for measuring the concentration of ions, especially H+ ions, in a measurement liquid. Measurement takes place by means of at least one ion sensitive field effect transistor (ISFET) which is located in the measurement liquid and which is connected together with the resistors in a bridge circuit. There is a bridge feed voltage at the feed points of the bridge circuit. The device delivers an output signal which is a measure of the ion concentration in the measurement liquid. The device has a reference electrode which is likewise located in the measurement liquid.
These devices are conventionally used to measure the pH of a measurement liquid. The pH is determined by the host of constituents dissolved in the measurement liquid, for example, by the concentration of H+ or OHxe2x88x92 ions.
In community and industrial waste water treatment, chemical reactions are used to precipitate certain constituents, to neutralize and detoxify the water. The pH plays a key role for the correct progression of these reactions. The mechanical settling processes in the clarification plants can be adversely affected by acid or alkali waste water. Biological processes in self-purification of water or in the aerobic and anaerobic biological stages of clarification plants are likewise linked to certain pH values and their progression is disrupted when there are deviations therefrom. To automate these operations continuous measurement of the pH is essential.
The concentration of H+ ions in a measurement liquid can be measured for example by means of an ion-sensitive field effect transistor (pH-ISFET), especially by means of a hydrogen ISFET. Compared to conventional glass electrode measurement chains, pH ISFETS have the advantage that they are not sensitive to aging and therefore have a much longer service life. The resistance of the channel of the pH-ISFET and thus the gate potential change linearly with respect to the concentration of H+ ions in the measurement liquid. So that the pH-ISFET delivers an output signal which is proportional to the input voltage on the pH-ISFET, there is a constant drain current on the pH-ISFET. The pH-ISFET delivers an output voltage as the output signal, for example. This output voltage is measured against a reference electrode which cannot be influenced by the concentration of H+ ions and which is likewise located in the measurement liquid.
EP 0 155 725 discloses a device of the initially mentioned type in which a pH-ISFET and a reference ISFET are connected together with two resistors in a bridge circuit. U.S. Pat. No. 4,334,880 discloses another device of the initially mentioned type, but without a reference electrode. The circuit described in the patent includes a pH-ISFET with three resistors being connected in a bridge circuit. The diagonal voltage of the bridge circuit is used as the output signal of the device. The conductivity of the pH-ISFET changes during the measurement process depending on the ion concentration in the measurement liquid, by which the measurement bridge is detuned and a diagonal voltage is formed.
Devices known in the prior art differ among one another especially in how the pH-ISFET is integrated into the circuitry of the device. In Analytical and Biomedical Applications of Ion-Selective Field Effect Transistors, P. Bergveld, A. Sibbald, Elsevier Science Publishers B.V. Amsterdam 1988, Chapter 8, ISFET instrumentation, pp. 101-107, different types of interconnection of a pH-ISFET in a device of the initially mentioned type are disclosed. FIG. 8.3 illustrates a circuit diagram which is detailed in section 8.5. Accordingly the pH-ISFET is integrated into an electrometer subtractor (with operational amplifiers A1, A2, A3) such that it is located in the circuit at the site of that resistor of the electrometer subtractor by which the gain of the electrometer subtractor can be adjusted. The electrometer subtractor has a power supply with a current source which delivers a constant current I and an adjustable reference voltage Vref. At the input of the electrometer subtractor on a resistor R1 there is a constant voltage Ixc3x97R1. The output signal of the electrometer subtractor delivers an output voltage which is inversely proportional to the resistance of the channel of the pH-ISFET. The output voltage is inverted by means of an inverter (A4). Finally, the difference between the inverted output voltage and the reference voltage Vref is amplified by means of an operational amplifier (A5). The output of the final operational amplifier (A5) is fed back to the input of the electrometer subtractor so that a feedback current flows via the resistor (R2). In this way the source and drain voltages of the pH-ISFET can be controlled. Based on this control the drain current (ID) and the drain source voltage (VDS) can be kept constant.
The connection of the pH-ISFET in an electrometer subtractor however has the disadvantage that this circuit is very complex to build. In particular, it requires a large number of components, for example three operational amplifiers (A1, A2, A3) and six ohmic resistances (R3, R4, R5, R6, R7, R8). Due to the large number of components the production of the circuit is time-consuming and complex. The known connection of the pH-ISFET is moreover extremely susceptible to temperature drift.
The object of this invention is therefore to embody and develop a device of the initially mentioned type such that a pH-ISFET is integrated in a circuit which is as simple as possible, which has a limited number of components, and which at the same time is not very susceptible to temperature drift and has high measurement accuracy.
To achieve this object the present invention proposes that at least one pH-ISFET with at least three resistors be connected in a bridge circuit, the diagonal voltage of the bridge circuit is between the p-input and the n-input of an operational amplifier, with an output which is fed back via two resistors of the bridge circuit to the inputs of the operational amplifier and the output signal of the device is formed as an output voltage which is formed from the difference of the drain potential of the pH-ISFET and the reference potential of the reference electrode.
In this way the pH-ISFET is integrated in a circuit of especially simple structure. The circuit consists of an extremely small number of components and can thus be produced economically. In spite of the simple structure of the circuit, the device of the present invention has all the features necessary to ensure proper operation of the pH-ISFET. Thus, the pH-ISFET operated in the circuit delivers, for example, an output signal which is proportional to the input voltage on the pH-ISFET. Moreover, the output signal is a reliable measure of the ion concentration in the measurement liquid and thus of the pH of the measurement liquid. There is preferably a linear relationship between the common logarithm of the H+ ion concentration in the measurement liquid and the output signal of the device.
The drain source voltage UDS on the pH-ISFET is set using the bridge feed voltage UBSS and the resistors R2 and R3 via which the output of the first operational amplifier is fed back to the inputs of the first operational amplifier. Here the following relationship applies:
UDS=UBSSxc3x97[R3/(R2+R3)]
The drain source current IDS can then be set by means of the drain source voltage UDS and of the resistor R1 via which the output of the first operational amplifier is fed back to the inputs of the first operational amplifier. Here the following relationship applies:
IDS=(UBSSxe2x88x92UDS)/R1
By means of these relationships the working point of the pH-ISFET can be easily set without changing the bridge feed voltage UBSS.
The drain potential linearly follows the change of the gate potential caused by the change of the pH, since the operational amplifier works as a P controller. The control circuit has the following transfer function:
∂xcfx86D/∂xcfx86G=1.
In the adjusted state of the circuit, the bridge is balanced, i.e. the diagonal voltage Ud=0. In the balanced state of the bridge, between the two series-connected resistors on one side of the bridge there is the same potential as between the resistor and the drain of the pH-ISFET on the other side of the bridge. Therefore, in the balanced state of the bridge, the output signal can also be tapped between the two series-connected resistors on one side of the bridge.
Each of the three resistors of the bridge circuit can of course also be replaced by a host of series-connected or parallel-connected resistors. The output signal can be tapped anywhere between these resistors. Depending on the size of the individual resistors, the output signal which has been tapped in this way is shifted by an offset voltage compared to the output signal tapped originally between the two series-connected resistors on one side of the bridge circuit or between the drain of the pH-ISFET and the resistor on the other side of the bridge circuit. In this way the zero point of the circuit (generally at pH 7) can be set independently of the other operating quantities of the pH-ISFET (especially the drain source voltage UDS and the drain source current IDS).
The output signal of the device is independent of temperature influences, since the device has a reference electrode which is likewise located in the measurement liquid. The output signal is formed as the output voltage which is formed from the difference of the drain potential of the pH-ISFET and the reference potential of the reference electrode.
According to one preferred development of the present invention, it is proposed that the device have a reference ISFET which is likewise located in the measurement liquid, the output signal being formed as an output voltage which is formed from the difference of the drain potential of the pH-ISFET and the drain potential of the reference ISFET. The reference ISFET is advantageously connected in a bridge circuit like the pH-ISFET of the device. According to this development, the conventional glass electrode for measuring the ion concentration is replaced by a pH-ISFET. Rather the reference electrode made as a conventional glass electrode is also replaced by an ISFET.
Advantageously the output of the first operational amplifier is routed back via a capacitor to the n-input of the first operational amplifier. In this way vibrations of the device can be suppressed.
According to one preferred embodiment of this invention the resistors are made as ohmic resistances. But it is also conceivable to replace the resistors via which the output of the operational amplifier is fed back to the inputs of the first operational amplifier by current sources. A current source can be built using a field effect transistor operated at saturation.
So that the device delivers an output signal which has light sensitivity as low as possible, according to one advantageous development of the invention it is proposed that the drain potential of the pH-ISFET and the pseudoreference potential of a potential reference electrode form a first difference signal, and the drain potential of the reference ISFET and the pseudoreference potential form a second difference signal, the output signal being formed as the difference of the first and the second difference signal, and the same light conditions prevailing on the gate regions of the pH-ISFET and the reference ISFET. The potential reference electrode is preferably made as a metal pin which is provided with a silver or a silver chloride coating. The reference ISFET is located in a measurement chamber with a constant pH (for example, pH=7). The measurement chamber is connected to the measurement liquid via a diaphragm.
In this device the pH-ISFET and the reference ISFET are operated in the so-called difference mode against the pseudoreference potential of the potential reference electrode. As a result of the difference formation the light-dependent signal portions of the pH-ISFET and of the reference ISFET, the two difference signals are almost completely compensated. Thus the output signal is also for the most part independent, under identical light conditions of the gate regions of the pH-ISFET. The reference ISFET is completely independent of the light conditions.
In this device, which is operated in the difference mode against the pseudoreference potential of the potential reference electrode, the pH ISFET together with at least three resistors is advantageously connected in a bridge circuit.
The bridge feed voltage is preferably constant. The constant bridge feed voltage is used by the operational amplifier of the circuit of the device of the present invention to produce an output voltage as stable as possible. The stability of the output voltage cannot be better than that of the bridge feed voltage. To keep the bridge feed voltage as stable as possible, it is important that the voltage source be as stable as possible.
There are various possibilities known for producing a constant voltage. According to one preferred embodiment it is proposed that the bridge feed voltage be formed as the energy gap voltage of a so-called bandgap diode.
Preferably the drain source voltage of the pH-ISFET is constant during the measurement process. In addition, or alternatively, the drain source current of the pH-ISFET is constant during the measurement process. The operating state in which both the drain source voltage and also the drain source current are constant is also called the constant charge mode (CCM). The gate of the ISFET represents a capacitor. By operating the ISFET in CCM, the capacitor charge remains constant, by which no recharging processes occur. In this way the ISFET can respond more quickly and it delivers a more accurate measured value since the hysteresis effect cannot occur.